As much as I'd jump on anything with new Nero content in it, I'm gonna have to third (fourth?) Zenos. Not just because he's awesome, but because the idea of he and Sephiroth interacting amuses the hell out of me.

Elidibus might be awkward to implement since he shares a seiyuu with Kuja, but that could be amusing in its own way...

For heroes I'd probably prefer to see Thancred as I both enjoy him as a character and think I'd like his playstyle...but more realistically I think it will be Lyse if anyone (ugh), with a slight chance of one or both of the twins (since they would both bring unique playstyles to the game, and would be efficient resource-wise since they're almost identical and even shared an outfit).